High temperature niobium base alloy



United States Patent man TEMPERATURE NIOBIUM BASE ALLOY Thor N. Rhodin, Jr., Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application September 27, 1957 Serial No. 686,597

11 Claims. (Cl. 75-174) This invention pertains to new, high-temperature niobium-base. alloys, and more particularly to alloys containing as essential elements niobium, titanium, molybdenum and chromium.

Workable alloys possessing strength and oxidation resistance at high temperatures have many industrial applications, and it is generally recognized that future development in many fields is closely tied to the availability of new and improved alloys possessing these properties. For example, the development of more advanced engines and gas turbines is dependent upon the availability of such alloys. Furthermore, there is a need for better, high-temperature alloys in the construction of dies for high-temperature working of metals and the construction of chemical reactors, oil refining equipment, and the like. A combination of workability, high-temperature strength, and high-temperature oxidation resistance in an alloy has proved diflicult to achieve, and considerable research has been undertaken in an effort to find such metals. I

It has now been found that workable alloys possessing unusually high oxidation resistance and strength at temperatures above 800 C. can be produced by alloying niobium, titanium, molybdenum, chromium (and if desired specified added elements) in the amounts set forth herein. It is, therefore, an object of this invention to Iprovide improved, workable alloys which possess unusually high oxidation resistance and strength at elevated temperatures. It is a further object to provide metal ,alloys suitable as materials of construction in high-temperature equipment of all types.

The alloy compositions inthe ensuing specification and claims are described in terms of weight percent, and

they comprise compositions containing about 120% titanium, about 1-20% molybdenum, about 1-16% chromium, and at least 50% niobium.

In addition to the above-named essential elements, the alloy may contain about 0-10% tantalum, about 010% vanadium, about 010% zirconium, about 0-5% alumiilum, about 0-5% cobalt, about 0-5% iron, about 05% manganese, about 0-5% nickel, about 05% tungsten, about 02% beryllium, about 0-2% carbon, about 0-2% -cerium, and about 02% silicon; the sum total of the group consisting of Ta, V, and Zr being about 0 to 20%; the sum total of the group consisting of Al, Co,'Fe, Mn,

"ice

Ni, and W being about 0 to 10%; and the sum total of the group consisting of Be, C, Ce, and Si being about 0 to 5%. a

In a preferred embodiment, the alloys of this invention comprise about 5-15 titanium, about 8-20% molybdenum, about 5-10% chromium and from about 55-75% niobium. The compositions of this preferred embodiment may also contain Be, C, Ce, Co, Fe, Mn, Ni, Si, Ta, V, W, Zr, and preferably A1, with or without Fe, in the amounts heretofore specified. Tungsten in amounts up to 5% is a preferred additional element.

The alloys of this invention can be prepared in accordance with conventional powder metallurgy techniques or by melting and casting. For example, weighed amounts of the individual metal constituents are melted together under an inert atmosphere, solidified and remelted until thorough mixing is obtained. The final melt is allowed to cool and solidify into a desired shape. The cast material thus obtained is a workable metal with strength and oxidation resistance at high temperatures and is suitable as a material of construction in high-temperature equipment. The melting operation can be carried out in an arc-melting furnace provided with consumable or nonconsumable electrodes; or by subjecting the charge to induction heating. A suitable arc-melting furnace is described by W. Kroll in Transactions of the Electrochemical Society, vol. 78, pages 35-47, 1940. This furnace has an integral, water-cooled copper crucible in which the charge can be melted and solidified. A continuous-feed type of furnace such as that described in U.S.P.B. Report 111,083, can also be used. Regardless of the type furnacing means employed, care should be exercised to protect the molten metal from normal atmospheric contamination through contact with oxygen, nitrogen, etc. This can be prevented by conducting the operation under a vacuum or an atmosphere of an inert .gas such as argon, helium, etc. The individual metals charged to the melting furnace can be in any desired form; for example, they may be in powder, granular, shot, wire or sponge form. H

For a clearer understanding of the invention, the following specific examples are given. These examples are intended to be merely illustrative of the invention and not in limitation thereof.

EXAMPLE I Each alloy of this example was prepared by placing the metal constituents which make up the final alloy composition in a water-cooled, copper crucible of an arc melting furnace of the type described in the above referred to Transactions of the Electrochemical Society The metals were then heated under an atmosphere-of helium until the charge became molten. The furnace was then turned off, and the melt was allowed to cool in the helium atmosphere. The alloy was thenmelted and solidified in this manner six more times to insure thorough mixing. The casting ultimately recovered was in the shape of the water-cooled coppercrucible.

A sample was taken from the casting and cheeked hy chemical analysis, and it was found that there was no maior'weight change in the composition as a result of the melting. Changes not greater than 5% based on the amount of the individual constituent are considered within tolerance.

A coupon or strip approximately x x "7 was cut from the casting with an abrasive wheel, and then machined to dimensions. This strip was heated for 24 hours at 1000 C. in a current of air flowing at the rate of 1000 cc. per minute. The heating was conducted in a recording thermobalance which continuously recorded the weight of the sample on a moving sheet of paper so that the weight change as a function of time is plotted on the paper. This plot makes it possible to determine the rate of oxidation at any given time during the heating period. There is given in the third column of Table I the rate of oxidation for the samples at the 24-hour point.

The fourth "colunmof Table I gives the weight percent of metal which has been converted to oxides. This percentage figure is obtained by removing the oxide scale from the sample and weighing the remaining alloy after the oxidation. The difference between this weight and the weight before oxidation is the weight of metal converted to oxides. Conversion of this weight difference to a pereentage figure is obtained by dividing by the weight of the unheated sample.

It is evident from the oxidation data given in the fourth column of Table I that molybdenum together with the other alloying elements improves the oxidation resistance of niobium. However, it is known that unalloyed molybdenum forms volatile oxide under the test conditions used herein, and for this reason, the gases from the heating zone were passed through a condenser so as to collect any form little or no volatile oxides is significant, since experience has shown that the use of molybdenum at the levels of this invention in high-temperature alloys can be limited by its susceptibility to form volatile oxides.

The samples were also measured for depth of surface recession and subscale thickness. These measurements were obtained by cutting through the samples following the oxidation test and then taking measurements on the cross-section. The depth of recession is the extent to which the oxidizing atmosphere has penetrated the face of the sample and converted the metal to external scale. For convenience, measurements are made from either of the larger faces. The depth of recession on these faces is substantially the same. By subscale" is meant that portion of the metal shown to be affected by the atmosphere, but not completely oxidized to external scale.

The information on edge-rating given in Table I is obtained by a visual grading to determine the degree to which the edges and corners of the sample have been rounded by the oxidizing atmosphere. In many uses of alloys of the present invention, it is important that machined or cast parts should retain relatively sharp edges and corners on exposure to oxidizing conditions at high temperatures. The use of rectangular, machined samples makes it possible to grade the alloy as to its ability to retain sharp edges and corners under severe oxidizing conditions.

The scale properties given in the last column of Table I were obtained from an examination of a cross section of the sample by metallography before the scale was removed. By such an examination, it is possible to see the degree to which the interior of the alloy is aifected by oxygen.

Table I Oxidation Wt. Percent Wt. Percent Composi- Rate in of Metal of Metal Depth of tion, mg./sq.cm./ Converted Converted Surface Subscale Edge- Percent hr. at the to Oxides to Volatile Recession Thickness Rating Scale Properties Wt. 24-111. after 24 Oxide after (cm.) (cm.)

Point hrs. at 24 hrs. at 1,000 G. 1,000 G.

Control.... 100% Nb. 98

Alloy 1....-- 0.9 15. 3 0 0.002 0. 0005 Excellent- Thin, brown, adherent.

N 0 internal degrada- Alloy 2 0. 8 l 2. 4 0 0.001 0. 0005 Good..... Do.

20% Mo-.- 5% Al. 23 P"- Alloy 3. 0.8 8.5 0 0.002 0. 0005 Excellent- Do.

Alloy 4 0.5 32 0.1 0.008 0.008 Fair Thick, brown, adherent. 20% gdalagginternal degra- 1 Oxidation of niobium proceeds at a high linear rate until substantially all of the metal is converted to oxide. The observed linear rate for niobium which compares with rate data for the other alloys in this table is 60.0 mg./sq. cm./hr.

Since niobium is so rapidly converted to oxide, the amount of metal available at the 24-hour point is so small that the oxidation rate at this point is meaningless.

1 The heating of this sample was extended to 120 hours.

At the end of this longer period, the oxidation rate was 0.9, and the percent weight change was 8.0. Another coupon composed of this alloy was heated for 24 hours at 1,200 C. under the conditions specified in this example, and the percent weight metal converted was 6.7.

volatile oxides which might form during the test. From the data presented in the fifth column of Table I, it will be seen that only one of the alloys of the table formed any volatile oxide and this amount was extremely small. This oxide was analyzed and found to be molybdenum oxide, and on the basis of this identification it is possible to calculate the weight percent of metal converted to yolatile oxide. The fact that the alloys of this invention EXAMPLE 'II This example is presented to show results obtained after heating for periods longer than the 24 hours as in Example I. The tests on oxidation resistance reported in Table II were conducted in the same manner as in Example I, except that the heating was for hours and the air flow rate was regulated between 1000-2000 cc. per minute.

; -This-example demonstrates the fabrieability of the alloys of this invention. A casting (prepared as in Example I from a charge comprising 75% niobium, 10% titanium, 10% molybdenum, and chromium) was hologenized by heating for 64 hours at 1400 C. in a vacuum furnace. The casting was first machined to a V2 inch round specimen and then swaged at 1300" C. to 0.422 inch, or approximately an 18% reduction without severe cracking. Another casting of the same alloy was homogenized as described above, swaged and machined into a discharge nozzle for molten salts, such as MgCl EXAMPLE IV Other outstanding high-temperature alloys made according to the procedure of Example I are as follows:

5% Ti, M0, 10% Cr, 5% Al, balance Nb 20% Ti, 10% Mo, 5% Cr, 1% Be, 5% Zr, balance Nb 10% Ti, Mo, 10% Ch, 8% Ta, 3% Ni, 1% Be,

balance Nb Ti, 15% Mo, 5% Cr, 3% Co, 1% C, balance Nb 15% Ti, 10% Mo, 16% Cr, 4% V, 2% Fe, 1% Be,

balance Nb 20% Ti, 10% MO, 10% Cr, 8% Ta, balance Nb 20% Ti, 10% Mo, 16% Cr, 3% Mn, balance Nb 10% Ti, 20% Mo, 8% Cr, 5% Co, 2% Mn, 1% Ce,

balance Nb 5% Ti, 5% Mo, 5% Cr, 7% Ta, 5% Zr, 5% Fe, 5% Al,

1% Be, 1% C, balance Nb 10% Ti, 10% MO, 10% C1, 10% V, 2% Mn, balance Nb 20% Ti, 20% Mo, 3% Cr, 3% Ni, 3% Al, balance Nb 10% Ti, 20% Mo, 10% Cr, 5%- Fe, 5%Al, balance Nb 15% Ti, 15% Mo, 5% Cr, 5% Al, 1% Ce, balance Nb 5% Ti, 20% Mo, 5% Cr, 5% W, 1% Ce, balance Nb 20% Ti, 5% Mo, 10% Cr, 5% W, balance Nb 10% Ti, 20% M0, 16% Cr, 1% Si, balance Nb 20% Ti, 10% Mo, 10% Cr, 1% C, balance Nb 10% Ti, 10% Mo, 10% Cr, 1% Ce, balance Nb It will be noted from Tables I and II above that the control samples of niobium compared very poorly with the alloys of this invention. Unalloyed niobium dissolves oxygen to a great extent at elevated temperatures. When niobium is exposed to an oxygen-containing atmosphere, an oxide film will form, and at elevated temperatures, oxygen from this film will difiuse into and dissolve in Table 11 Wt. Hardness Percent 01 OomposiofMetal Depth of Subscale "As-cast" tlon. Converted Surface Thlckness Edge- Button: Percent to Oxides Recession (cm.) Rating (Brtnell Wt. after (cm.) Hardness Hrs. at Number) 1,000 o.

,l [77% Nb.-. Alloye 21.0 0.025 0.020 mn-...... m 107 o... 82%N 4110 0 El 20.0 0.020 0.000 f81l'.----.- s09 67 0.--- 6 oNb.--

M10 1 0 ool'.--- 9.5 0.025 0.028 mun..." 285 15% Mo.-. ass-- If 109510.111 Alle s as 0.022 0.070 good e10 1% Al 00% Nb-.. 15% TL--- Alle s i'g 7.s 0.020 0.001 good s01 EXAMPLE III 30 the metal until the latter is saturated with oxygen, thus destroying the ductility of the metal and rendering it unsuitable for practical use. Although the reason for the unique properties of the alloys of this invention are not completely understood, it is believed that they are the result of two phenomena which will be described below. However, it should be understood that the invention is in no way limited by this explanation. In the alloy of this invention, elements, such as titanium, which have greater affinities for oxygen than does niobium are used. These elements will combine with oxygen and constitute a second phase in the matrix. Thus, deleterious oxygen is consumed by the formation of stable compounds as it attempts to penetrate into the structure, and as a result the diffusion of oxygen is slowed down so that the oxygen concentration remains low in the bulk of the alloy. The second phenomenon appears to result from the use of solid-solution elements, such as molybdenum, which decrease the difiusion and solubility of oxygen in the alloy matrix. Therefore, in the alloys of this invention there are two phenomena whose combined eifect is to slow down and reduce the amount of oxygen which would otherwise find its way into the bulk of the metal and destroy is mechanical properties.

Although it is preferred to employ metals exhibiting relatively high purity, some variance in purity can be tolerated. Thus, the alloys of the examples and those tested were obtained from commercially available metals containing less than 1% incidental impurities. Commercial niobium usually contains tantalum (in amounts up to 5%) which is difiicult to quantitatively detect so that it is considered for practical purposes as niobium. Therefore, the niobium contemplated for use herein may contain small amounts (up to 5%) of tantalum, as well as incidental impurities (such as iron, oxygen, carbon, and silicon) common to the metal.

The alloys of this invention may be used as a material of construction in any structure which requires a strong, oxidation-resistant metal. They are particularly suitable for use in high-temperature equipment, such as engines, turbines, chemical reactors and their accessories, oil refining equipment, and high-temperature dies. However, it should be emphasized that the use of these alloys is not limited to high-temperature conditions or to any piece of equipment described herein. Moreover, it is 7 pointed out that these alloys possess high resistance to corrosive materials, such as hot mineral acids and fused salts.

Since it is obvious that many changes and modifications can be made in the above-described details without departing from the nature and spirit of the invention, it is to be understood that the invention is not to be limited to said details except as set forth in the appended claims,

I claim:

1. An alloy consisting essentially of, by weight, about l-20% titanium, about 120% molybdenum, about 116% chromium, at least 50% niobium, about -10% tantalum, about 010% vanadium, about 0-10% zir conium, about 0-5% aluminum, about 05% cobalt, about 05% iron, about 05% manganese, about 05% nickel, about 05% tungsten, about 02% beryllium, about 02% carbon, about 02% cerium, and about 02% silicon; the sum total of Ta, V, and Zr being about 020%; the sum total of A1, C0, Fe, Mn, Ni, and W being about 0-10%; and the sum total of Be, C, Ce, and Si being about 05 2. An alloy consisting essentially of, by weight, about 5-15 titanium, about 820% molybdenum, about 5-10% chromium, about 55-75% niobium, about 0-10% tantalum, about 0-l0% vanadium, about 010% zir-' conium, about 05% aluminum, about 05% cobalt, about 05% iron, about 05% manganese, about 05% nickel, about 05% tungsten, about 02% beryllium, about 02% carbon, about 02% cerium, and about 02% silicon; the sum total of Ta, V, and Zr being about 0-20%; the sum total of A1, C0, Fe, Mn, Ni, and W being about 0-l0%; and the sum total of Be, C, Ce, and Si being about 0-5 3. An alloy consisting essentially of, by weight, about 1-20% titanium, about 1-20% molybdenum, about 1-10% chromium, the balance being essentially niobium.

4. An alloy consisting essentially of, by weight, about 1-20% titanium, about 1 -20% molybdenum, about 1-10% chromium, up to 5% aluminum, the balance being essentially niobium.

5. An alloy consisting essentially of, by weight, about l20% titanium, about 1-20% molybdenum, about 110% chromium, up to 5% aluminum, up to 5% iron, the balance being essentially niobium.

6. An alloy consisting essentially of, by weight, about l-20% titanium, about 1-20%- molybdenum, about 1-10% chromium, up to 5% tungsten, the balance being essentially niobium. I

7. An alloy consisting essentially of, by Weight, about 10% titanium about 10% molybdenum, about 3% chromium, the balance being essentially niobium.

8. An alloy consisting essentially of, by weight, about 10% titanium, about 20% molybdenum, about 10% chromium, about 5% aluminum, about 5 iron, the balance being essentially niobium,

9. An alloy consisting essentially of, by weight, about 5% titanium, about 20% molybdenum, about 5% chromium, about 5% tungsten, about 1% Cerium,- the balance being essentially niobium.

10. An alloy consisting essentially of, by weight, about 10% titanium, about 20% molybdenum, about 10% chromium, about 5% aluminum, the balance being essentially niobium.

11. An alloy consisting essentially of, by weight, about 10% titanium, about 15% molybdenum, about 5% chromium, about 1% cerium, the balance being essentially niobium.

No references cited. 

1. AN ALLOY CONSISTING ESSENTIALLY OF, BY WEIGHT, ABOUT 1-20% TITANIUM, ABOUT 1-20% MOLYBDENUM, ABOUT 1-16% CHROMIUM, AT LEAST 50% NIOBIUM, ABOUT 0-10% TANTALUM, ABOUT 0-10% VANADIUM, ABOUT 0-10% ZIRCONIUM, ABOUT 0-5% MANANGESE, ABOUT 0-5% COBALT ABOUT 0-5% IRON, ABOUT 0-5% MANGANESE, ABOUT 0-5% NICKEL, ABOUT 0-5% TUGSTEN, ABOUT 0-2% BERYLLIUM, ABOUT 0-2% CARBON, ABOUT 0-2% CERIUM, AND ABOUT 0-2% SILICON; THE SUM TOTAL OF TA, V AND ZR BEING ABOUT 0-2; THE SUM TOTAL OF AL, CO, FE, MN, NI, AND W BEING ABOUT 0-10%; AND THE SUM TOTAL OF BE,C, CE, AND SI BEING ABOUT 0-5%. 